Screw pump

ABSTRACT

A first rotor and a second rotor have helical teeth each including a dedendum portion, an addendum portion, and an addendum tip portion. Profiles of the addendum portions of the first and second rotors are formed by cycloidal curves. Profiles of the dedendum portions of the first and second rotors are formed by trochoidal curves. A pitch circle of the second rotor is larger than that of the first rotor. The number of teeth of the second rotor is larger than that of the first rotor. A width angle of the first rotor is not less than a minimum angle at which the addendum tip portions of the first rotor are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is approximately equal to that at the minimum angle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a screw pump.

2. Related Background Art

A screw pump is provided with a first rotor and a second rotor having helical teeth. The first rotor and the second rotor mesh with each other, thereby forming a working chamber. The screw pump is configured to rotate the first rotor and the second rotor, so as to confine a fluid in the working chamber and transfer the fluid from an inlet to an outlet. It is known that the screw pump reduces its transfer efficiency of the fluid due to a blow-hole area or the like. For this reason, an improvement in efficiency is achieved by devising the geometries of the rotors, for example, as described in Japanese Patent Application Laid-open No. 2001-73959.

The Laid-open No. 2001-73959 discloses the screw pump in which a driving rotor (the first rotor) and a driven rotor (the second rotor) mesh with each other in a rotatable state. In the screw pump disclosed in the Laid-open No. 2001-73959, a main tooth profile of the driving rotor consists of a cycloid drawn by an addendum (tooth top) tip of the driven rotor. A main tooth profile of the driven rotor consists of a trochoid drawn by an addendum tip of the driving rotor. This prevents occurrence of blow-by of fluid (the blow-hole).

SUMMARY OF THE INVENTION

Incidentally, since the addendum tips of the driven rotor are acute angles in the screw pump disclosed in the Laid-open No. 2001-73959, there is a problem of requiring considerable man-hours for processing steps and for ensuring quality. Furthermore, the screw pump generally has another problem of poor volumetric efficiency when compared to roots vacuum pumps. The rotors occupy a large volume relative to a rotor housing volume, resulting in a small discharge volume. For this reason, the screw pump has the size larger than the roots vacuum pumps and others with the same capacity.

An object of the present invention is to provide a screw pump reduced in acute-angled portions of rotors and achieving a high discharge volume ratio.

An aspect of the present invention is a screw pump comprising: a housing in which an inlet and an outlet are formed; a first rotor having helical teeth each including a first dedendum portion (tooth root portion), a first addendum portion (tooth top portion), and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing, wherein profiles of the first and second addendum portions are formed by cycloidal curves and profiles of the first and second dedendum portions are formed by trochoidal curves, wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor, wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.

In the present invention, the profiles of the first addendum portions are formed by the cycloidal curves and the profiles of the first dedendum portions are formed by the trochoidal curves. For this reason, the face width of the first rotor is narrow enough to reduce a rotor occupancy ratio which is a ratio of the rotor volume to the cylinder volume. Therefore, it is feasible to improve the discharge volume ratio of the screw pump. Furthermore, since obtuse-angled portions are formed at the second addendum tip portions of the second rotor, the acute-angled portions can be reduced when compared to the conventional screw pumps. The discharge volume ratio is a ratio obtained by multiplying a theoretical discharge volume ratio, which is a ratio of a theoretical discharge volume to the cylinder volume, by a volumetric efficiency resulting from clearances between the housing and the rotors.

The screw pump may be configured as follows: on the pitch circles where the teeth of the first rotor and the teeth of the second rotor mesh with each other, the cycloidal curve of the first addendum portion varies to the trochoidal curve of the first dedendum portion and the cycloidal curve of the second addendum portion varies to the trochoidal curve of the second dedendum portion.

The width angle of the first rotor may be within 4° from an angle at which the discharge volume ratio is a maximum. Furthermore, the width angle of the first rotor may be not less than the minimum angle and not more than an angle 9° larger than the minimum angle.

The screw pump may be configured as follows: a ratio of an interval between a rotation axis of the first rotor and a rotation axis of the second rotor to a diameter of an outer circle of the first rotor is in the range of not less than a minimum of an establishment limit and not more than a value 0.02 larger than the minimum.

The screw pump may be configured as follows: the first rotor has three teeth, and the second rotor has four teeth.

Another aspect of the present invention is a screw pump comprising: a housing in which an inlet and an outlet are formed; a first rotor having helical teeth each including a first dedendum portion, a first addendum portion, and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing, wherein profiles of the first addendum portions are formed by cycloidal curves and profiles of the first dedendum portions are formed by involute curves, wherein profiles of the second addendum portions are formed by involute curves and profiles of the second dedendum portions are formed by trochoidal curves, wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor, wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.

In the present invention, the profiles of the first addendum portions are formed by the cycloidal curves, the profiles of the first dedendum portions are formed by the involute curves, the profiles of the second addendum portions are formed by the involute curves, and the profiles of the second dedendum portions are formed by the trochoidal curves. For this reason, the face width of the first rotor is narrow enough to reduce the rotor occupancy ratio. Therefore, it is feasible to improve the discharge volume ratio of the screw pump. Furthermore, obtuse-angled portions are formed at the second addendum tip portions of the second rotor, whereby the acute-angled portions can be reduced when compared to the conventional screw pumps.

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top cross-sectional view of a screw pump according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a first rotor and a second rotor according to the embodiment.

FIG. 3 is a cross-sectional view showing the first rotor and second rotor used in simulations.

FIG. 4 is a drawing showing the simulation result.

FIG. 5 is a drawing showing the simulation result.

FIG. 6 is a drawing showing the simulation result.

FIG. 7 is a drawing showing the simulation result.

FIG. 8 is a cross-sectional view of rotors of a screw pump according to a modification example of the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description, the same elements or elements with the same functionality will be denoted by the same reference signs, without redundant description.

As shown in FIG. 1, screw pump 10 is a horizontally-installed screw pump. The screw pump 10 is used, for example, as an oil-free vacuum pump. A housing of the screw pump 10 is composed of a rotor housing 11, a front housing 12 joined to the front end portion of the rotor housing 11, and a rear housing 13 joined to the rear end portion of the rotor housing 11. A first rotor 20 and a second rotor 30 meshing with each other are housed in a space inside the housing.

An inlet 14 for intake of a fluid into the housing is formed at one end of the rotor housing 11 (on the left side in FIG. 1). An outlet 15 for discharge of the fluid in the housing to the outside is formed at the other end of the rotor housing 11 (on the right side in FIG. 1). The inlet 14 has a nearly rectangular opening and is arranged nearer to the second rotor 30. The inlet 14 faces an engaging position of the two rotors 20, 30. The outlet 15 opens on the side of the second rotor 30. The opening area of the outlet 15 is set smaller than that of the inlet 14.

A first shaft 21 penetrates the first rotor 20 and is fixed to the first rotor 20. A second shaft 31 penetrates the second rotor 30 and is fixed to the second rotor 30. The rotation axis A1 of the first rotor 20 and the rotation axis A2 of the second rotor 30 are arranged in parallel at an interval L. One end portion of the first shaft 21 (on the left side in FIG. 1) is passed through an axial hole 12A formed in the front housing 12 and is supported through a bearing 18 on the front housing 12. The other end portion of the first shaft 21 (on the right side in FIG. 1) is passed through an axial hole 13A formed in the rear housing 13 and is supported through a bearing 18 on the rear housing 13.

One end portion of the second shaft 31 (on the left side in FIG. 1) is passed through an axial hole 12B formed in the front housing 12 and is supported through a bearing 18 on the front housing 12. The other end portion of the second shaft 31 (on the right side in FIG. 1) is passed through an axial hole 13B formed in the rear housing 13 and is supported through a bearing 18 on the rear housing 13. Namely, the first rotor 20 is rotatably supported in a both-ends supported state on the housing by the first shaft 21 projecting from the two ends thereof. The second rotor 30 is rotatably supported in a both-ends supported state on the housing by the second shaft 31 projecting from the two ends thereof.

On the opposite side to the rotor housing 11 with respect to the rear housing 13, a gear housing 40 is joined to the rear housing 13. The gear housing 40 forms a gear chamber 41 together with the rear housing 13. The other end portion of the first shaft 21 penetrates the rear housing 13 and is fixed to a driving gear 42 in the gear housing 40. The other end portion of the second shaft 31 penetrates the rear housing 13 and is fixed to a driven gear 43 in the gear housing 40. An electric motor 45 as a drive source is arranged in the gear housing 40 and an output shaft 46 of the electric motor 45 is coupled through a coupling 47 to the other end portion of the first shaft 21. The driving gear 42 meshes with the driven gear 43, whereby rotation of the first shaft 21 is transmitted through the driving gear 42 and the driven gear 43 to the second shaft 31, thereby achieving synchronous rotation of the first rotor 20 and the second rotor 30.

Next, the geometries of the first rotor 20 and the second rotor 30 will be described on the basis of FIG. 2.

The first rotor 20 is a three-teeth male rotor. The first rotor 20 has an axial periphery represented by an inner circle 20B1. The first rotor 20 is provided with a plurality of teeth (three teeth in the present embodiment) 20A radiating in radial directions from the axial periphery and having a helical shape. The inner circle 20B1 is a circle centered on the rotation axis A1 of the first shaft 21. The teeth 20A, as shown in FIG. 2, are arranged at equal intervals in a cross section of the first rotor 20.

Each tooth 20A of the first rotor 20 has a dedendum portion (tooth root portion) 20A1 located on the rotation axis A1 side, and an addendum portion (tooth top portion) 20A2 located on the outer periphery side of the dedendum portion 20A1. A boundary between the dedendum portion 20A1 and the addendum portion 20A2 is located on a pitch circle of the first rotor 20 represented by a medium circle 20B2. Each tooth 20A has an addendum (tooth top) tip portion 20A3 represented by an outer circle 20B3, at a radial distal end of the addendum portion 20A2. As shown in FIG. 2, the dedendum portion 20A1 is a portion located between the inner circle 20B1 and the medium circle 20B2 in the radial direction. The addendum portion 20A2 is a portion located between the medium circle 20B2 and the outer circle 20B3 in the radial direction. The diameter of the medium circle 20B2 is set to be larger than that of the inner circle 20B1. The diameter of the outer circle 20B3 is set to be larger than that of the medium circle 20B2. A portion of an inner periphery of the rotor housing 11 is positioned along the outer circle 20B3. The addendum tip portion 20A3 is in contact with the inner periphery of the rotor housing 11 on the outer circle 20B3. Since the first rotor 20 has the teeth 20A formed in the helical shape, the addendum tip portions 20A3 are in line contact with the inner periphery of the rotor housing 11.

The second rotor 30 is a four-teeth female rotor. The second rotor 30 has an axial periphery represented by an inner circle 30B1. The inner circle 30B1 is a circle centered on the rotation axis A2 of the second shaft 31. The second rotor 30 is provided with a plurality of teeth (four teeth in the present embodiment) 30A radiating in radial directions from the axial periphery and having a helical shape. The teeth 30A mesh with the teeth 20A of the first rotor 20. The teeth 30A, as shown in FIG. 2, are arranged at equal intervals in a cross section of the second rotor 30.

Each tooth 30A of the second rotor 30 has a dedendum portion 30A1 located on the rotation axis A2 side, and an addendum portion 30A2 located on the outer periphery side of the dedendum portion 30A1. A boundary between the dedendum portion 30A1 and the addendum portion 30A2 is located on a pitch circle of the second rotor 30 represented by a medium circle 30B2. Each tooth 30A has an addendum tip portion 30A3 represented by an outer circle 30B3, at a radial distal end of the addendum portion 30A2. As shown in FIG. 2, the dedendum portion 30A1 is a portion located between the inner circle 30B1 and the medium circle 30B2 in the radial direction. The addendum portion 30A2 is a portion located between the medium circle 30B2 and the outer circle 30B3 in the radial direction. The addendum tip portion 30A3 is formed in an arcuate shape so as to extend along the outer circle 30B3. The diameter of the medium circle 30B2 is set to be larger than that of the inner circle 30B1. The diameter of the outer circle 30B3 is set to be larger than that of the medium circle 30B2. In the present embodiment, the outer circle 20B3 of the first rotor 20 and the outer circle 30B3 of the second rotor 30 have the same diameter. A portion of the inner periphery of the rotor housing 11 is positioned along the outer circle 30B3. Each addendum tip portion 30A3 has a certain surface in contact with the inner periphery of the rotor housing 11 on the outer circle 30B3.

Profiles of the dedendum portions 20A1 of the first rotor 20 are formed by trochoidal curves based on the outer circle 30B3 of the second rotor 30. Namely, the profile of each dedendum portion 20A1 is formed as a trochoidal curve drawn from a given point on the medium circle 20B2 being the pitch circle of the first rotor 20, by an arbitrary point on the outer circle 30B3 of the second rotor 30. In the present embodiment, the profiles of the dedendum portions 20A1 all are trochoidal curves. Profiles of the addendum portions 20A2 of the first rotor 20 are formed by cycloidal curves based on the medium circle 30B2 of the second rotor 30. Namely, the profile of each addendum portion 20A2 is formed as a cycloidal curve drawn from a given point on the medium circle 20B2 being the pitch circle of the first rotor 20, by an arbitrary point on the medium circle 30B2 of the second rotor 30. Two cycloidal curves intersect with each other at the addendum tip portion 20A3 (on the outer circle 20B3). The profiles of the addendum portions 20A2 all are cycloidal curves.

The face width of each tooth 20A of the first rotor 20 is defined by a width angle θ shown in FIG. 2. The width angle θ is an angle made between lines connecting the width on the medium circle 20B2 of the tooth 20A from the rotation axis A1 of the first rotor 20. In the first rotor 20, the three teeth 20A are formed each with the same width angle θ. If the width angle θ is larger than the angle shown in FIG. 2, the addendum tip portions 20A3 will expand so as to be in face contact with the rotor housing 11 on the outer circle 20B3. If the width angle θ is smaller than the angle shown in FIG. 2, the addendum tip portions 20A3 will be located radially inside so as not to be in contact with the rotor housing 11. For this reason, the fluid will leak through the gap and the screw pump 10 will fail to function as a pump. The width angle θ in the state in which the addendum tip portions 20A3 are in line contact with the inner periphery of the rotor housing 11 on the outer circle 20B3 as shown in FIG. 2 is a minimum angle of the width angle θ. Namely, the width angle θ in the state in which the addendum tip portions 20A3 are in line contact with the outer circle 20B3 is the minimum angle of the width angle θ. In the present embodiment, the width angle θ of the first rotor 20 is set at this minimum angle.

Profiles of the dedendum portions 30A1 of the second rotor 30 are formed by trochoidal curves based on the outer circle 20B3 of the first rotor 20. Namely, the profile of each dedendum portion 30A1 is formed as a trochoidal curve drawn from a given point on the medium circle 30B2 being the pitch circle of the second rotor 30, by an arbitrary point on the outer circle 20B3 of the first rotor 20. In the present embodiment, the profiles of the dedendum portions 30A1 all are trochoidal curves. Profiles of the addendum portions 30A2 of the second rotor 30 are formed by cycloidal curves based on the medium circle 20B2 of the first rotor 20. Namely, the profile of each addendum portion 30A2 is formed as a cycloidal curve drawn from a given point on the medium circle 30B2 being the pitch circle of the second rotor 30, by an arbitrary point on the medium circle 20B2 of the first rotor 20. In the present embodiment, the trochoidal curves of the dedendum portions 30A1 are in contact with the inner circle 30B1 of the second rotor 30. Namely, two adjacent teeth 30A have their respective dedendum portions 30A1 formed by one trochoidal curve. The addendum portions 30A2 are formed by the cycloidal curves up to portions reaching the inner periphery of the rotor housing 11 (outer circle 30B3).

Now, let us explain the simulation results about the width angle θ of the first rotor 20 and the theoretical discharge volume ratio in the screw pump 10. FIG. 3 is a drawing showing a cross section of the first rotor 20 and the second rotor 30 of the screw pump 10 used in the simulations below. In the simulations, the diameter of the outer circle 20B3 of the first rotor 20 is 100 millimeters. The interval L between the rotation axis A1 of the first rotor 20 and the rotation axis A2 of the second rotor 30 is 70 millimeters. The first rotor 20 is provided with three teeth and thus the number of teeth is 3. The second rotor 30 is provided with four teeth and thus the number of teeth is 4. In the first rotor 20 and the second rotor 30, the profiles of the dedendum portions 20A1, 30A1 are formed by trochoidal curves. The profiles of the addendum portions 20A2, 30A2 are formed by cycloidal curves.

FIG. 4 shows a change of theoretical discharge volume ratio in the screw pump 10, with change in the width angle θ of the first rotor 20 shown in FIG. 3. The theoretical discharge volume ratio herein is defined by Formula (1) below.

Theoretical discharge volume ratio=theoretical discharge volume/cylinder volume  (1)

The theoretical discharge volume is a discharge volume per complete rotation of the first rotor 20. The cylinder volume is the volume of the rotor housing 11 in which the first rotor 20 and the second rotor 30 are housed.

As shown in FIG. 4, the simulation was conducted with the width angle θ of the first rotor 20 being set at each of 51°, 60°, 70°, and 80°. As a result, the theoretical discharge volume ratio decreased with increase in the width angle θ of the first rotor 20. According to this simulation result, the theoretical discharge volume ratio is the largest, about 0.50, when the width angle θ is 51°. Therefore, the width angle θ of the first rotor 20 is preferably set as small as possible. In the conditions of this simulation, the minimum angle of the width angle θ of the first rotor 20 is 51°. Namely, if the width angle θ is set below 51°, the first rotor 20 will have a gap relative to the inner periphery of the rotor housing 11 so as to increase the fluid leakage, thereby resulting in significant reduction of efficiency of the screw pump 10.

FIG. 4 shows the simulation result of the change of theoretical discharge volume ratio in the screw pump 10, with change in the width angle θ of the first rotor 20, in a state in which the addendum tip portions 20A3 and the addendum tip portions 30A3 are in contact with the inner periphery of the rotor housing 11 on the outer circle 20B3, 30B3. In actual manufacture of the screw pump 10, however, clearances are needed between the addendum tip portions 20A3 of the first rotor 20 and the rotor housing 11 and between the addendum tip portions 30A3 of the second rotor 30 and the rotor housing 11 because of friction resistance and manufacturing tolerance. Since these clearances change the volumetric efficiency, a real discharge volume ratio is defined by Formula (2) below.

Discharge volume ratio=theoretical discharge volume ratio×volumetric efficiency  (2)

FIG. 5 shows a change of discharge volume ratio in the screw pump 10, with change in the width angle θ of the first rotor 20 shown in FIG. 3, in a configuration in which ordinary clearances in the size of the screw pump 10 are set. As shown in FIG. 5, the simulation was conducted with the width angle θ of the first rotor 20 being set at each of 51°, 60°, 70°, and 80°. As a result, the discharge volume ratio once increased and then decreased with increase in the width angle θ of the first rotor 20. According to the simulation result, the discharge volume ratio is about 0.461 at the width angle θ of 51° and about 0.460 at the width angle θ of 60°. A simulation was conducted to check the discharge volume ratio in more detail with the width angle θ being set between 51° and 60°. The discharge volume ratio reached a maximum at the width angle θ of 55° and then decreased at the width angles θ of more than 55°. The discharge volume ratios in the range of width angle θ of up to 59° are approximately equal to that at the width angle θ of 51°. In the conditions of this simulation, the minimum of the width angle θ of the first rotor 20 is also 51°.

Next, the result of another simulation will be described on the basis of FIG. 6. The conditions of the present simulation are the same as the simulation conditions in FIG. 4. Namely, the diameter of the outer circle 20B3 is 100 millimeters and the interval L 70 millimeters. Then the theoretical discharge volume ratio was measured with change in each of the number of teeth of the first rotor 20 and the number of teeth of the second rotor 30. The width angle θ of the first rotor 20 was always set at the minimum angle even with change in the number of teeth.

As shown in FIG. 6, when the number of teeth of the first rotor 20 is 3, the theoretical discharge volume ratio is lower with the number of teeth of the second rotor 30 being 5 than that with the number being 4. When the number of teeth of the first rotor 20 is 4, the theoretical discharge volume ratio is lower with the number of teeth of the second rotor 30 being 6 than that with the number being 5. It is seen about the numbers of teeth of the first rotor 20 and the second rotor 30 from this result that the theoretical discharge volume ratio can be made large when the difference between the number of teeth of the first rotor 20 and the number of teeth of the second rotor 30 is the smallest. Under the conditions of the present simulation, where the number of teeth of the first rotor 20 was 5, the second rotor 30 could not be formed with the number of teeth being 7.

Furthermore, as shown in FIG. 6, the theoretical discharge volume ratio increases with increase in the number of teeth of the first rotor 20 from 3 to 4 and 5. According to this simulation result, the theoretical discharge volume ratio is the highest, about 0.535, when the number of teeth of the first rotor 20 is 5 and the number of teeth of the second rotor 30 is 6.

Next, the result of another simulation will be described on the basis of FIG. 7. In the present simulation, the width angle θ of the first rotor 20 was always set at the minimum angle. The diameter of the outer circle 20B3 is 100 millimeters. Then a change of theoretical discharge volume ratio was measured with change in the numbers of teeth of the first rotor 20 and the second rotor 30 and with change in the interval L between the rotation axis A1 of the first rotor 20 and the rotation axis A2 of the second rotor 30. The number of teeth of the second rotor 30 is set to be larger than the number of teeth of the first rotor 20 and the difference between them is set to the smallest. Namely, the number of teeth of the second rotor 30 is one larger than that of the first rotor 20.

As shown in FIG. 7, the theoretical discharge volume ratio increases with decrease in the interval L from 70 millimeters. In the present simulation result, the theoretical discharge volume ratio is the largest, about 0.63, when a ratio of the interval L to the outer circle 20B3 (interval L/diameter of outer circle 20B3) is 0.62, in the configuration wherein the number of teeth of the first rotor 20 is 3 and that of the second rotor 30 is 4.

In the configuration wherein the number of teeth of the first rotor 20 is 3, if the ratio of the interval L to the outer circle 20B3 is smaller than 0.62, the first rotor 20 and the second rotor 30 will become too close to each other. For this reason, the screw pump 10 cannot be established. Namely, an establishment limit of the screw pump 10 about the ratio of the interval L (62 mm) to the outer circle 20B3 (100 mm) is 0.62 in the configuration wherein the number of teeth of the first rotor 20 is 3 and that of the second rotor 30 is 4. The ratio of the interval L to the outer circle 20B3 can be at least 0.62 being the establishment limit. Similarly, in the configuration wherein the number of teeth of the first rotor 20 is 4 and that of the second rotor 30 is 5, a minimum value as an establishment limit about the ratio of the interval L (66 mm) to the outer circle 20B3 (100 mm) is 0.66.

In the configuration wherein the number of teeth of the first rotor 20 is 5 and that of the second rotor 30 is 6, a minimum value as an establishment limit about the ratio of the interval L (69 mm) to the outer circle 20B3 is 0.69. Therefore, it is seen from the simulation result shown in FIG. 7 that the ratio of the interval L to the outer circle 20B3 is preferably set in the range from the establishment limit value of the first rotor 20 and the second rotor 30 to a value 0.02 larger than the establishment limit value, with a high theoretical discharge volume ratio.

In light of the simulation results shown in FIGS. 4 to 7, the geometries of the first rotor 20 and the second rotor 30 shown in FIG. 2 will be further described. The number of teeth of the first rotor 20 is 3. The number of teeth of the second rotor 30 is larger than that of the first rotor 20 and is 4 with the smallest difference in the number of teeth. The diameter of the outer circle 20B3 of the first rotor 20 is set at 100 mm. The interval L between the rotation axis A1 of the first rotor 20 and the rotation axis A2 of the second rotor 30 is preferably set at 62 mm. The width angle θ of the first rotor 20 is preferably set between 51° and 59°. Although the width angle θ is preferably set between 51° of the minimum angle and 59°, approximately 1° can be a permissible range in view of manufacturing tolerance. Namely, the width angle θ can be not less than 51° and not more than 60°, with a high discharge volume ratio. The width angle θ of the first rotor 20 is preferably set within 4° from the angle at which the discharge volume ratio is the maximum. Since the rotor housing 11 has no involvement in the simulations shown in FIGS. 6 and 7, an increase in theoretical discharge volume ratio leads directly to an increase in discharge volume ratio.

The present embodiment achieves the effects described below.

In the first rotor 20 and the second rotor 30 of the screw pump 10, the profiles of the respective dedendum portions 20A1, 30A1 are formed by the trochoidal curves and the profiles of the respective addendum portions 20A2, 30A2 are formed by the cycloidal curves. This reduces the blow-hole area and thus enables provision of high-efficiency screw pump 10.

The second rotor 30 has the number of teeth larger than that of the first rotor 20 and the pitch circle of the second rotor 30 is set larger than that of the first rotor 20. Since the width angle θ of the first rotor 20 is set at an angle to achieve the discharge volume ratio approximately equal to that at the minimum angle at which the addendum tip portions 20A3 are in line contact with the outer circle 20B3, the screw pump 10 can have a high discharge volume ratio. Since the discharge volume ratio is high, the screw pump 10 can be constructed in a reduced size.

Since the second rotor 30 is formed by the cycloidal curves and trochoidal curves, acute-angled portions of the second rotor 30 are reduced. The reduction in acute-angled portions facilitates processing. Furthermore, it improves the quality of the second rotor 30.

Since in the first rotor 20 the three teeth 20A are provided at equal intervals in the circumferential direction and radiate in radial directions from the axial periphery, the first rotor 20 is well-balanced during rotation. Since in the second rotor 30 the four teeth 30A are provided at equal intervals in the circumferential direction and radially in radial directions from the axial periphery, the second rotor 30 is well-balanced during rotation. Then the screw pump 10 is well-balanced during rotation of the first rotor 20 and the second rotor 30 in the intermeshing state.

The ratio of the interval L between the rotation axis A1 of the first rotor 20 and the rotation axis A2 of the second rotor 30 to the outer circle 20B3 of the first rotor 20 is set in the range of not less than the minimum of the establishment limit and not more than the value 0.02 larger than the minimum. This can enhance the theoretical discharge efficiency of the screw pump 10. Furthermore, it permits reduction in the size of the screw pump 10.

It should be noted that the present invention is by no means limited to the above embodiment. In the above embodiment the profiles of the first rotor 20 and the second rotor 30 are formed by only the cycloidal curves and trochoidal curves, but the present invention is not limited to this example. For example, as shown in FIG. 8, profiles of dedendum portions 60A1 of first rotor 60 may be formed in part by involute curves. FIG. 8 is a cross-sectional view of a screw pump in which the first rotor 60 with four teeth 60A having the width angle θ meshes with a second rotor 70 with six teeth 70A. Each tooth 60A has a dedendum portion 60A1 located between an inner circle 60B1 and a medium circle 60B2, an addendum portion 60A2 located between the medium circle 60B2 and an outer circle 60B3, and an addendum tip portion 60A3 being a tip of the addendum portion 60A2. Each tooth 70A has a dedendum portion 70A1 located between an inner circle 70B1 and a medium circle 70B2, an addendum portion 70A2 located between the medium circle 70B2 and an outer circle 70B3, and an addendum tip portion 70A3 being a tip of the addendum portion 70A2.

The profiles of the dedendum portions 60A1 of the first rotor 60 are formed by involute curves and trochoidal curves. The profiles of the dedendum portions 60A1 are the involute curves from the medium circle 60B2 toward the inner circle 60B1 and transfer to the trochoidal curves near the inner circle 60B1. The profiles of the addendum portions 70A2 of the second rotor 70 are formed by cycloidal curves and involute curves. The involute curves in the dedendum portions 60A1 of the first rotor 60 correspond to those in the addendum portions 70A2 of the second rotor 70. As the first rotor 60 and the second rotor 70 rotate in synchronization, the involute curves go into contact with each other. The screw pump can also be realized with a large discharge volume ratio and in a reduced size in the configuration wherein the involute curves are adopted for the profiles of the first rotor 20 and the second rotor 30. The width angle θ of the first rotor 60 is set at an angle at which the discharge volume ratio of the screw pump is approximately equal to that at the minimum angle.

The screw pump shown in FIG. 8, which is not shown, has the housing composed of the rotor housing 11, the front housing 12, and the rear housing 13 as the screw pump 10 shown in FIG. 1 does. The first rotor 60 and the second rotor 70 are housed in the space inside the housing. Profiles of the addendum portions 60A2 of the first rotor 60 are formed by cycloidal curves as the profiles of the addendum portions 20A2 of the first rotor 20 are. Profiles of the dedendum portions 70A1 of the second rotor 70 are formed by trochoidal curves as the profiles of the dedendum portions 30A1 of the second rotor 30 are. The pitch circle of the second rotor 70 is set larger than that of the first rotor 60. On the pitch circles where the teeth 60A of the first rotor 60 and the teeth 70A of the second rotor 70 mesh with each other, the cycloidal curve of the addendum portion 60A2 varies to the involute curve of the dedendum portion 60A1 and the involute curve of the addendum portion 70A2 varies to the trochoidal curve of the dedendum portion 70A1. The ratio of the interval L between the rotation axis A1 of the first rotor 60 and the rotation axis A2 of the second rotor 70 to the outer circle 60B3 of the first rotor 60 is set in the range of not less than the minimum of the establishment limit and not more than the value 0.02 larger than the minimum. The width angle θ of the first rotor 60 is within 4° from the angle at which the discharge volume ratio is a maximum. The width angle θ of the first rotor 60 is not less than the minimum angle and not more than an angle 9° larger than the minimum angle.

The profiles of the dedendum portions 20A1, 30A1 and the addendum portions 20A2, 30A2 do not have to be formed completely by the cycloidal curves or by the trochoidal curves. For example, the teeth 20A, 30A may be formed by partially modifying arcuate curves, near the addendum tip portions 20A3, 30A3. The medium circles 20B2, 30B2 do not always have to be on the pitch circles. The medium circles 20B2, 30B2 can be larger or smaller than the respective pitch circles, while achieving a large theoretical discharge volume ratio and a reduction in size of the screw pump.

In the above embodiment the outer circle 20B3 of the first rotor 20 and the outer circle 30B3 of the second rotor 30 have the same size, but the present invention is not limited to this example. The outer circles 20B3, 30B3 have the same size in the configuration of the present embodiment shown in FIG. 2, but the sizes of the respective outer circles 20B3, 30B3 may be varied depending upon applications and places for use of the screw pump 10.

From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 

1. A screw pump comprising: a housing in which an inlet and an outlet are formed; a first rotor having helical teeth each including a first dedendum portion, a first addendum portion, and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing, wherein profiles of the first and second addendum portions are formed by cycloidal curves and profiles of the first and second dedendum portions are formed by trochoidal curves, wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor, wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.
 2. The screw pump according to claim 1, wherein on the pitch circles where the teeth of the first rotor and the teeth of the second rotor mesh with each other, the cycloidal curve of the first addendum portion varies to the trochoidal curve of the first dedendum portion and the cycloidal curve of the second addendum portion varies to the trochoidal curve of the second dedendum portion.
 3. The screw pump according to claim 1, wherein the width angle of the first rotor is within 4° from an angle at which the discharge volume ratio is a maximum.
 4. The screw pump according to claim 1, wherein the width angle of the first rotor is not less than the minimum angle and not more than an angle 9° larger than the minimum angle.
 5. The screw pump according to claim 1, wherein a ratio of an interval between a rotation axis of the first rotor and a rotation axis of the second rotor to a diameter of an outer circle of the first rotor is in the range of not less than a minimum of an establishment limit and not more than a value 0.02 larger than the minimum.
 6. The screw pump according to claim 1, wherein the first rotor has three teeth, and wherein the second rotor has four teeth.
 7. A screw pump comprising: a housing in which an inlet and an outlet are formed; a first rotor having helical teeth each including a first dedendum portion, a first addendum portion, and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing, wherein profiles of the first addendum portions are formed by cycloidal curves and profiles of the first dedendum portions are formed by involute curves, wherein profiles of the second addendum portions are formed by involute curves and profiles of the second dedendum portions are formed by trochoidal curves, wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor, wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.
 8. The screw pump according to claim 7, wherein on the pitch circles where the teeth of the first rotor and the teeth of the second rotor mesh with each other, the cycloidal curve of the first addendum portion varies to the involute curve of the first dedendum portion and the involute curve of the second addendum portion varies to the trochoidal curve of the second dedendum portion.
 9. The screw pump according to claim 7, wherein the width angle of the first rotor is within 4° from an angle at which the discharge volume ratio is a maximum.
 10. The screw pump according to claim 7, wherein the width angle of the first rotor is not less than the minimum angle and not more than an angle 9° larger than the minimum angle.
 11. The screw pump according to claim 7, wherein a ratio of an interval between a rotation axis of the first rotor and a rotation axis of the second rotor to a diameter of an outer circle of the first rotor is in the range of not less than a minimum of an establishment limit and not more than a value 0.02 larger than the minimum.
 12. The screw pump according to claim 7, wherein the first rotor has four teeth, and wherein the second rotor has six teeth. 